1. Field of the Invention
This invention relates to a semiconductor controlled rectifier which can control a large amount of electric power and effect a very rapid switching action.
2. Description of the Prior Art
A typical example of a conventional semi-conductor controlled rectifier (hereinafter referred to for brevity as SCR) comprises a semiconductor substrate having four alternate p- and n-type layers, a pair of main electrodes kept in ohmic contact with the outermost p- and n-layers and a control electrode kept in contact with one of the two intermediate layers.
With an SCR having such a structure as described above, when a control signal voltage in the form of a pulse is applied between the control electrode and one of the main electrodes (hereinafter referred to as first main electrode) which is in ohmic contact with the outermost layer (hereinafter referred to as first outermost layer) adjacent to the intermediate layer (hereinafter referred to as first intermediate layer) provided with the control electrode, with a forward voltage applied between the first main electrode and the remaining main electrode, i.e. second main electrode, the SCR turns on and current flows through the two main electrodes for the SCR. The SCR of this type is turned off, that is, switched from its conducting state to its non-conducting state, by inverting the polarity of the voltage applied between the first and the second main electrodes. There are, however, many cases where the SCR's must be turned off with the polarity of the forward voltage remaining unchanged. In such cases, the SCR is turned off by applying such a control signal voltage as to reverse-bias the PN junction between the first outermost layer (therefore the remaining outermost layer being referred to hereinafter as second outermost layer) and the first intermediate layer (therefore the remaining intermediate layer being referred to hereinafter as second intermediate layer), that is, between the control electrode and the first main electrode. This turn-off function will be described. The SCR in its conducting state is left under the condition that a great number of majority and minority carriers are injected into its layers, particularly the intermediate layers. Under this condition, if a control signal voltage to reverse-bias the PN junction between the first outermost layer and the first intermediate layer is applied between the control electrode and the first main electrode, the injection of the carriers from the first outermost layer into the first intermediate layer is blocked in the close vicinity of the control electrode and the carriers in the intermediate layers flow out of the control electrode. Accordingly, there exists a turn-off area only in the close vicinity of the control electrode. In the region remote from the control electrode, the control signal voltage has no strong influence upon the carriers since the first intermediate layers has a large lateral resistance. It is, therefore, impossible in that region to drain the carriers through the control electrode, to reverse-bias the PN junction between the first outermost layer and the first intermediate layer and therefore to block the injection of the carriers from the first outermost layer into the first intermediate layer. This means that the regions remote from the control electrode still remain conductive even after the application of the control signal voltage for turn-off, so that the SCR can not be switched into its turn-off state. It is necessary for turn-off of the SCR by using only the control signal voltage applied to the control electrode that the control signal voltage be high enough or that the conduction region of the SCR have a small cross-sectional area. With a high control signal voltage, the SCR can indeed be turned off even if the cross-sectional area of the conductive region is relatively large, but the influence of the control signal is still small in the regions remote from the control electrode, and consequently there is a drawback that the time required for establishing the turn-off state becomes adversely longer and then there is another drawback that the power required for turn-off action is large because of lower efficiency. On the other hand, if the cross-sectional area of the conduction region is small, the current capacity becomes adversely smaller.
As described above, with a conventional SCR, heavy current cannot be cut off at high speed and with high efficiency.